Retaining walls and coastal protection walls are required to sustain all weather conditions and large stresses over long periods. It is also a requirement in many instances that the wall retain an attractive appearance or at the least not be a prominent defacement to the surrounding environment.
Timber has been used for constructing retaining walls. However, timber has the disadvantages of splintering and decaying over time. Furthermore, timber walls typically require the use of chemical preservatives like creosote, nickel or arsenic which are potentially hazardous both to the environment and the handlers of these chemicals. Quality timber is in high demand world-wide and supply is limited and therefore alternative materials are sought which can give an economic and environmental advantage over timber.
Cast-in-place concrete and conventional mortared masonry walls have also been used for constructing retaining walls. These are inflexible and are susceptible to stresses created by movement and settlement of the material being retained and foundations of the wall. Furthermore, water may be trapped behind walls of this type, resulting in the development of hydrostatic pressure behind the wall. This pressure may place further strain on the wall, perhaps shortening its life span. Furthermore, walls of this type may be unattractive in many surroundings as they do not readily allow plants to grow over the surface of the wall. Cast-in-place walls require a cast to be constructed in the right position and means to get the concrete slurry into the cast. This can be difficult to achieve in areas where access is limited.
Segmental retaining walls have been used widely for centuries. However, many wall constructions have traditionally required internal steel reinforcement. Corrosive expansion of the reinforcing steel or other metal could exceed the capacity of the block to contain the extra volume, weakening the blocks and the wall which they form. Also, traditional segmental retaining walls may allow water to pass through the wall only between the gaps in the blocks or not at all, creating hydrostatic pressure. Also, steel reinforcement can protrude from any grassed upper wall surface causing problems to motor mowers.
U.S. Pat. No. 5,865,006 (Dawson) discloses a concrete retaining wall block. However this is not of a permeable concrete.